Recently, with the advancement in optical technologies, organic fluorescent compounds have attracted attentions in a wide range of fields such as light-emitting materials for organic electroluminescence, dye lasers, and bioimaging. The organic fluorescent compounds are particularly applicable to a color conversion technique in which light at a desired wavelength is emitted after absorption at a specific wavelength. The practical applications as a light conversion film have been actively studied for various uses in organic electroluminescent displays, solar cells, liquid crystal displays, agricultural films, light-emitting diodes, and so forth.
For example, Patent Literature 1 discloses a diarylamine derivative useful as an organic electroluminescent element or an electrophotographic photoreceptor. Patent Literature 2 discloses a polyester resin useful in an organic electroluminescent element. Further, Patent Literature 3 discloses a diamine compound polymer having condensed aromatic groups and applicable to organic electronic devices. Patent Literature 4 discloses a triarylamine-based polymer and a polymeric light emitting element using the same.
Furthermore, Patent Literature 5 discloses a light-diffusing resin composition containing a dye having a maximum absorption wavelength in a range of 470 nm to 520 nm or 570 nm to 610 nm and a maximum fluorescence wavelength in a range of 380 nm to 470 nm, 520 nm to 570 nm, or 610 nm to 700 nm, and an image display using the composition.
Furthermore, Patent Literature 6 discloses a color converter characterized by being made of a fluorescent substance which absorbs a short wavelength component of incident light and converts the short wavelength component into a long wavelength component for emission, and a plant growing apparatus including the color converter disposed therein. Patent Literature 7 discloses a light conversion film and an image display using an organic fluorescent substance employable in a light-emitting diode.
Researches for practical applications of organic EL elements have been actively conducted. Organic EL elements can achieve a high current density at a low voltage, and thus is expected to achieve high luminance and luminous efficiency. Particularly, the application to multicolor organic EL displays capable of displaying high-definition multiple colors or full color is expected. As one example of a method of creating a multicolor or full-color organic EL display, there is a method using multiple types of color filters each transmitting light in a specific wavelength region (color filter method). When the color filter method is employed, an organic EL element to be used is required to emit beams of multiple colors to thereby emit so-called “white light” containing three primary colors of light (red (R), green (G), blue (B)) in a good balance.
As another method of obtaining a multicolor emission organic EL element, there has been proposed a color conversion method using a single-color emission organic EL element and a color conversion film (see, for example, Patent Literature 8). The color conversion film used therein is a layer containing one or more color conversion substances each of which absorbs light of a short wavelength and converts the light into light of a long wavelength.
When a color conversion film is formed using a single color conversion substance, however, a so-called concentration quenching phenomenon occurs where an absorbed energy is deactivated without emitting light, as the energy is repeatedly transferred between the same molecules. To suppress this phenomenon, a medium dispersion-type color conversion film is proposed in which the concentration of a color conversion substance is lowered by being dissolved or dispersed in a certain medium to thus suppress the contact between molecules of the color conversion substance (see, for example, Patent Literature 9).
To form a color conversion film by a coating process carried out at low cost, the color conversion substance is required to be soluble. Moreover, from the viewpoint of the process, desirably, the viscosity adjustment suitable for a device to be used is easily made. To achieve such a purpose, it is desirable to use a color conversion substance made of a polymeric material which is dissolvable in a wide range of solvents, and which allows easy adjustment of the solution viscosity by changing the molecular weight.
Conventionally, as the color conversion film using a polymeric material, a color conversion film containing a conjugated polymeric material has been proposed. Specifically, there has been proposed a color conversion film containing a polyarylenevinylene derivative (see, for example, Patent Literature 10) or a polyfluorene derivative (see, for example, Patent Literature 11) which are known as light-emitting materials for polymeric EL.